1. Field of the Invention
The present invention generally relates to electronic devices. More particularly, this invention relates to reducing the amount of stray light that can enter an optical sensing module.
2. Description of the Related Art
A variety of applications exist and continue to be developed for electronic devices that operate on the basis of optics. For example, image sensors have been developed for use in automotive applications to sense the presence of vehicle occupants and objects in the vicinity of a vehicle. An example of such a sensor is represented in FIG. 1, which depicts a module 10 comprising a lens assembly 12 containing a pair of lenses 14, a housing 16 in which the lens assembly 12 is installed, a glass substrate 18 to which the housing 16 is affixed, and an imager chip 20 with one or more appropriate sensing elements and circuitry for sensing light that has passed through the lenses 14 and the glass substrate 18. Photons impinging the imager chip 20 are converted by the sensing elements into charge carriers (i.e. electron/hole pairs), so that light is electrically detected. As known in the art, the sensing elements may be individual detectors or arrays of detectors, as in CMOS or CCD imager arrays. While two lenses 14 are used in the module 10, it is foreseeable that fewer or more lenses could be employed in optical path, and a lens may not be required for certain imaging applications.
The chip 20 is physically attached to a chip carrier 22 with electrically-conductive (e.g., solder or conductive adhesive or Au/Cu stud bump) connections 24, which also electrically connect the chip circuitry to conductors 26 on the chip carrier 22. The chip carrier 22 can be a flexible substrate laminated to the glass substrate 18. As an example, the chip carrier 22 can be a flexible circuit (also referred to as a flex circuit), which as known in the art is a flexible substrate of an electrically insulating material such as a polyimide or polyester film, often in the form of a flat cable, with patterned conductors (signal lines) along its length. The chip 20 is underfilled on the chip carrier 22 with an underfill material 28 to promote the reliability of the connections 24. The underfill material 28 also fills a gap 30 between the chip 20 and glass substrate 18. Because light sensed by the sensing elements on the chip 20 must pass through the underfill material 28 and gap 30, the underfill material 28 is formed of an optically-matching material, preferably an adhesive, that fully fills the gap 30 without voids and allows unimpeded passage of light.
For use in its application, the module 10 may be used without any enclosure, or placed in an enclosure that minimizes the amount of light leaking in from the environment. From FIG. 1, it can be appreciated that light can enter the module 10 through the glass substrate 18 and the optically-matching underfill material 28, as well as the chip carrier 22 if formed of conventional flex circuit materials. Infrared light can also enter through the chip 20 if formed of a material such as silicon. For modules ultimately housed in an enclosure, it is advantageous to be able to test the modules as they are produced, prior to assembly into enclosures, since their enclosures may or may not be light-impermeable. Modules used alone without an enclosure must be capable themselves of preventing stray light from being detected by the enclosed imager chip 20.
In view of the above, it can be appreciated that it would be desirable to reduce the amount of light that can enter an optical sensing module at the module level.